Some of the optically active compounds, (3R,5S)-3,5,6-trihydroxyhexanoic acid derivatives having the specific configuration shown by following formula (1') ##STR1## (wherein R.sup.1' and R.sup.4 each represents a hydrogen atom or a protective group for a hydroxy group and R.sup.2' represents an ester forming group), are already known compounds and it is reported that these compounds can be easily converted into lactone moieties of a chemical structure of compactin, mevinolin, etc., which are given attention as an antihyperlipemia as described in K. Prasad et al., Tetrahedron Lett., Vol. 25, No. 32, 3391-3394(1984). It is considered that the lactone moiety is the active part of an inhibitor on 3-hydroxy-3-methylglutarylcoenzyme A (hereinafter, is referred to as "HMG-CoA") reductase, one of the predominant enzymes taking part in the increase of the concentration of cholesterol, which is the cause of hyperlipemia and many analogs having the lactone moiety have been synthesized [J. R. Prous, Drugs of the Future, Vol. 12, No. 5, 437-442(1987)].
Many methods of synthesizing the (3R,5S)-3,5,6-trihydroxyhexanoic acid derivatives shown by foregoing formula (1') have been reported. In these methods, as a method originated in a raw material compound having asymmetric carbon atoms at two portions, there is a method of synthesizing from D-glucose which is a natural optically active compound as described in T. Lee, Tetrahedron Lett., Vol. 26, No. 41, 4995-4996(1985) but the method has a disadvantage that the reaction step up to obtained the desired compound is long.
Also, as a method of simultaneously introducing hydroxy groups at two portions into a syn-diol form, there is a method of obtaining a 3,5-dioxohexanoic acid derivative by reacting an acetoacetic acid derivative and an amide compound and reducing the reaction product using sodium borohydride and an alkoxydialkylborane at a temperature from -70.degree. C. to -78.degree. C. as described in JP-A-1-165547 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and T. Hanamoto et al., Tetrahedron Lett., Vol. 29, No. 49, 6467-6470(1988) but by the method, a diastereomer only is determined but a desired optically active material can not be obtained.
A method of forming asymmetric carbon atoms at two portions one by one has been most widely utilized and as the methods, following methods (a) to (g) are reported.
(a) A method of obtaining an (S)-5,6-dihydroxy-3-oxohexanoic acid derivative shown by following formula (2') ##STR2## wherein R.sup.1' and R.sup.4 each represents a hydrogen atom or a protective group for a hydroxy group and R.sup.2' represents an ester forming group, from an (S)-3,4-dihydroxybutanoic acid ester derivative shown by following formula (5) ##STR3## wherein R.sup.1' and R.sup.4 have the same meaning as described above and R.sup.5 represents a hydrogen atom or an alkyl group, synthesized from L-malic acid, which is a natural material, and diastero-selectively reducing the foregoing hexanoic acid derivative using a reducing agent such as a mixture of sodium borohydrate and a trialkylborane or a mixture of sodium borohydrate and an alkoxydialkylborane under a low temperature condition of about -70.degree. C. [European Patent 0 244 364 A2, U.S. Pat. No. 4,970,313, G. Wess et al., Tetrahedron Lett., Vol. 31, No. 18, 2545-2548(1990), and K. Prasad et al., Tetrahedron: Asymmetry, Vol. 1, No. 5, 307-310(1990)].
(b) A method of obtaining the compound shown by foregoing formula (2') by reacting an optically active butyronitrile derivative and an .alpha.-haloacetic acid ester in the presence of zinc and diastero-selectively reducing the reaction product as in step (a) described above (U.S. Pat. No. 4,983,759).
(c) A method of synthesizing the compound shown by formula (2') from an optically active 3-hydroxypropionaldehyde derivative and diastero-selectively reducing the synthesized product as in step (a) described above [M. Fukui et al., Chem. Pharm. Bull., Vol. 38, No. 10, 2890-2892(1990)].
(d) A method of synthesizing the compound shown by formula (2') from 3-(3-methoxyphenyl)-2-propen-1-ol and then diastero-selectively reducing the synthesized compound as in step (a) [D. A. Evans et al., J. Org. Chem., Vol. 56, No. 2, 741-750(1991)].
In addition, the diastero-selective reducing reaction of compound shown by formula (2') common to the steps (a) to (d) is also reported in K. Chen et al., Tetrahedron Lett., Vol. 28, No. 2, 155-158(1987), K. Chen et al., Chemistry Lett., 1923-1926(1987), etc. Also, in F. G. Kathawala et al., Helv. Chim. Acta, Vol. 69, 803-805(1986), the diastero-selectivities for the compound shown by foregoing formula (1') obtained in the case of using sodium borohydrate and a trialkylborane, the case of using ferrous chloride, the case of using zinc borohydride, etc., for the diastero-selective reducing reaction of the compound shown by formula (2') are compared. Furthermore, it is reported that in the compound shown by formula (1') having an allyl group or a cyano group in place of the hydroxy group at the 6-position, which is an analogous compound to the foregoing compound shown by formula (1'), by applying the diastero-selective reducing reaction to the substrate obtained by carrying out, for example, an asymmetric aldol condensation of the aforesaid compound, and then optically resolving the reaction product, a corresponding optically active 3,5-dihydroxyhexanoic acid derivative can be obtained [J. E. Lynch et al., Tetrahedron Lett., Vol. 28, No. 13, 1385-1388(1987); N. Balasubramanian et al., J. Med. Chem., Vol No. 9, 2038-2041(1989); S. Y. Sit et al., J. Med. Chem., Vol. 33, No. 11, 2982-2999(1990), U.S. Pat. No. 5,003,080].
(e) A method of obtaining the compound shown by formula (2') by reacting the dianion of acetoacetic acid ester with aldehyde and reducing the compound by reacting the compound using sodium borohydrate in absolute ethanol under ice-cooling (U.S. Pat. No. 4,248,889).
(f) A method of obtaining the compound shown by formula (5) by ring-cleaving optically active 1-benzyloxy-2,3-epoxy-4-hydroxybutane and after introducing a protective group to the hydroxy group, oxidizing the product, and synthesizing by carrying out a Reformatsky reaction of the compound of formula (5) using ethyl bromoacetate and zinc [K. Prasad et al., Tetrahedron Lett., Vol. 25, No. 32, 3391-3394(1984)].
(g) A method of reacting a (3R,5R)-5-amino-3-hydroxyhexanoic acid derivative and a specific cyan compound [Na.sub.2 Fe(CN).sub.5 NO] using potassium carbonate [G. J. McGarvey et al., J. Org. Chem., Vol. 51, No. 20, 3913-3915(1986)].
However, the above-described methods each has the following defect.
That is, the method (e) is a method of obtaining a racemic modification and a desired optically active substance can not be obtained.
In the method (f), it is difficult to obtain the optically active epoxide which is the raw material, and also the Reformatsky reaction does not have the selectivity for the desired syn-diol form.
In the method (g), it is difficult to obtain the raw material and also the yield for the desired compound is low.
The diastereo-selective reducing reaction which is utilized for methods (a) to (d), which are mostly reported, is a method of inducing an asymmetric point at another portion by utilizing the asymmetric point of one portion and in methods (a) to (d), a desired compound can be obtained with a considerably high syn-diol form selectivity as compared with methods. (e) to (g). However, in methods (a) to (d), the reaction must be carried out under a low temperature condition, whereby a specific equipment is required in the case of synthesizing in an industrial scale, and also in these methods, an expensive reagent is required. Furthermore, in the method (a), L-malic acid which is the raw material is a very expensive compound.
As a method for improving these faults, a method of carrying out an enantioselective hydrogenation of a 4-tert-butoxyacetoacetic acid ester, which can be synthesized at a low cost, as a raw material using a ruthenium-optically active phosphine complex as a catalyst to form an (S)-4-tert-butoxy-3-hydroxybutyric acid ester, reacting the butyric acid ester thus obtained and the lithium enolate of an acetic acid ester to provide an (S)-6-tert-butoxy-5-hydroxy-3-oxohexanoic acid ester, and obtaining the desired compound shown by formula (1') by diastereo-selectively enantioselectively hydrogenating the product thus obtained under a mild temperature condition using a ruthenium-optically active phosphine complex as a catalyst is disclosed in U.S. Pat. No. 994,602.
However, although the method can overcome the foregoing defects in the points of the raw material and the reaction condition, the method is not sufficiently satisfactory in the point that the diastereo-selectivity of the hydroxy group at the 3-position of the compound of formula (1') obtained is not so high as from 60% d.e. to 82% d.e.
In addition, in the foregoing methods, in the methods of carrying out the diastereo-selective reaction, R.sup.4 of the compound shown by formula (2') is a hydrogen atom in all cases. Accordingly, R.sup.4 of the compound shown by formula (1') obtained is also a hydrogen atom. In other methods, the compounds shown by formula (1') wherein R.sup.4 is a hydrogen atom or a protective group for a hydroxy group are synthesized but the protective groups for the hydroxy group practically disclosed are a 3-substituted silyl group and a phenylaminocarbonyl group only, which are reluctant to deprotect, from the reasons of requiring a very expensive reagent and a severe reaction condition.
Thus, the development of a method of simply and efficiently obtaining a (3R,5S)-3,5,6-trihydroxyhexanoic acid derivative the hydroxy group at the 5-position of which can be easily deprotected has been desired.